This disclosure relates generally to wireless communication systems, and in particular but not exclusively, relates to use of a tapered fiber bundle (TFB) for high-power coupling at a single transmission aperture optical terminal in a free space optical communication system.
Current techniques for wavelength division multiplexing (WDM) of channels for transmission of optical signals through free space have their limitations and disadvantages. For example, signals are multiplexed onto a single fiber prior to free space transmission. WDM devices to implement this multiplexing are low-power devices that generally require multiplexing to occur prior to power amplification. This pre-amplification multiplexing in turn requires implementation of a single high-power amplifier that is capable to provide all of the power required for a WDM system. Use of the single high-power amplifier generates problems associated with non-linear effects within the high-power amplifier, and within the fiber between the high-power amplifier and a transmit aperture. Overly high-powered signals in an optical amplifier and in downstream optical fiber will cause distortion, such as 4-wave mixing, which will cause crosstalk among the optical channels, and thus result in bit errors at a receive terminal. Thus when high optical power levels per wavelength are required, only a limited number of wavelengths can be amplified together without introducing severe performance penalties.
WDM systems sometimes use a multiple-IN/single-OUT coupler just before the transmit aperture, with one or more high-power amplifiers at an input side of the coupler amplifying the signals that are input into the coupler. However, use of such a coupler results in significant power loss on the output side of the coupler. For example, a 4-to-1 coupler results in a 6-dB (or 3/4) drop in power, after amplification. Therefore, the cost of the amplification is borne, and then power is discarded (e.g., much of the amplification provided by the high-power amplifier is xe2x80x9cthrown awayxe2x80x9d by the coupler for the sake of obtaining the single output).
WDM systems sometimes use fiber-based WDM couplers that combine separate wavelength bands together with minimal optical insertion loss. However, these couplers can have problems handling extremely high optical power levels (e.g., multiple watts). Even fiber connectors can have reliability problems with the extremely high power levels that would result from combining a large number of high-power optical amplifier outputs.
In some instances, an optical terminal with multiple transmit apertures may be used as a relatively simple solution to multiplexing. However, the optical terminal has increased complexity in both design and co-alignment of the optical transmit apertures to each other and to a receive aperture at the receive terminal.
In addition to these limitations and disadvantages, the various implementations described above are costly when reliability and scalability are considered. Therefore, improvements are needed in the transmission of optical signals.
According to an aspect of the invention, an apparatus includes a first plurality of single-mode optical fibers into which to inject wavelength division multiplexed (WDM) optical signals. A tapered fiber bundle (TFB) is coupled to the first plurality of single-mode optical fibers, with the TFB including a second plurality of fused single-mode optical fibers correspondingly coupled to the first plurality of single-mode optical fibers at a first end of the TFB. The second plurality of fused single-mode optical fibers are capable to receive the WDM optical signals from the first plurality of single-mode optical fibers at the first end and to provide the WDM optical signals to a free space optical communication path at a second end.